More precisely, the invention concerns applying desired time offsets between various unit signals that respectively modulate various spectrum components of a light beam to be processed. The invention is particularly applicable to the following typical case: initial overall modulation has been applied to an initial light beam so as to cause it carry information to be transmitted. That modulation has caused initial unit signals to appear which modulate the spectrum components of the beam. Finally, the unit signals have undergone various interference time offsets, such interference time offsets typically resulting from the interference chromatic dispersion that is specific to a medium through which the light of the initial beam propagated to constitute the beam to be processed. The present invention then aims to process the beam so as to apply desired chromatic dispersion thereto. The desired chromatic dispersion is chosen so as to compensate for the interference chromatic dispersion to the extent that is necessary to enable the information which was to be transmitted to be yielded from a processed beam.
In particular, the invention is advantageously applicable to following more specific case, which, in practice, is of considerable importance.
Prior to the present invention, a long transmission line (longer than 100 km) has been laid in the form of a line optical fiber equipped with intermediate optical amplifiers or repeaters for transmitting information at medium or high data rates. That prior fiber (already laid) typically has very high chromatic dispersion, in the vicinity of 20 ps/nm.km. Technical progress now makes it possible to transmit at very high data rates (10 Gbits/s) over a fiber with no intermediate optical amplifier or repeater. But the very high chromatic dispersion of the already-laid prior fiber prevents this possibility from being used if such very high data-rate transmission is to achieved over that fiber.
The data rate being limited due to the high dispersion may be avoided by performing modulation outside the source, on transmission, thereby reducing spreading of the spectrum of the source, which spreading would otherwise result from the source being modulated directly by its injection current (chirp or interference frequency modulation due to intensity modulation).
However, the spectrum occupancy of the transmitted optical signal also results from the data-rate of the information to be transmitted. For links having very high data-rates (10 Gbits/s), the transmitted signal takes up a spectrum range corresponding to the modulation sidebands, i.e. about 0.2 nm. The chromatic dispersion results in total dispersion for the link of about 400 ps over 100 km, which is incompatible with the transmission data rate of 10 Gbits/s.
Apparatus for compensating the chromatic dispersion of the line fiber is therefore necessary if that prior fiber is to be used.
Such compensation may be performed, at the reception end, by a first known compensation apparatus constituted by an optical fiber that is much shorter than the line fiber but that has very high dispersion of the opposite sign. The optimum value of the compensation depends not only on the dispersion of the line fiber, but also on the non-linear effects that appear in that fiber. Said optimum value enables compensation to be performed effectively. But the space required by the compensation fiber is a drawback if the fiber is to be disposed in the form of a coil integrated in the receiver terminal.
A particular object of the present invention is to reduce the space required by dispersion compensation apparatus situated in the receiver terminal of a very long optical link.
A second known compensation apparatus includes firstly a color-splitting system for separating a plurality of spectrum components of the beam to be processed into physically distinct positions, and secondly a plurality of optical fibers of various lengths for respectively transmitting the components.
The second known apparatus is described in Document U.S. Pat. No. 3,863,063 (G. S. Indig et al).
Positioning the plurality of fibers of the second apparatus is difficult, and coupling losses appear at the inputs and outputs of the fibers.
A particular object of the present invention is to avoid using such a plurality of optical fibers.
In a third known dispersive optical apparatus, a first diffraction grating receives a parallel beam to be processed, and angularly separates the spectrum components of the beam. The angularly separated spectrum components are received by a second diffraction grating which is said to be "conjugate" with the first diffraction grating because it makes the components parallel again while conserving their respective separate positions. A mirror that is perpendicular to the beam then returns them to the second grating which returns them to the first grating which reconstitutes a processed parallel beam that propagates in the opposite direction from the beam to be processed, and that is separated therefrom by a sloping semi-reflective plate. The differences in path length of the various spectrum components achieve the desired chromatic dispersion.
In the third known apparatus, the set of two gratings constitutes both a color-splitting system, in the go direction, and also a color-recombining system, in the return direction. The desired time offsets are achieved inside that set.
Such known dispersive apparatus is described in the following article: TUP4 Compensation of negative group velocity dispersion in optical fibers with a grating and telescope pulse compressor. Anatoly Frenkel, Jonathan P. Heritage, Oscare. Martinez.--(CLEO '88/Tuesday Afternoon/130-133)--published by the Optical Society of America--2010 Massachusetts Avenue, NW, Washington D.C. 20036-1023.
That apparatus suffers from the drawback that the time offsets that can be applied are limited and insufficient for compensating the chromatic dispersion of a typical very long line fiber.